This invention relates, in general, to electron beam lithography and is particularly directed to the reduction in pattern distortion during beam writing on a workpiece in an electron beam lithography machine.
In an electron beam lithography machine, there is a beam column in which an electron beam is electronically and magnetically controlled and projected onto a workpiece which is mounted on a movable stage to perform a writing operation on the workpiece. The workpiece also called a substrate and may be a resist coated wafer or a resist coated mask located below the beam column.
When the electron beam bombards the substrate, the beam produces high energy backscattered electrons which are reflected upwardly and outwardly from the impact point and which are sensed by electron detector diodes located near the exit of the beam column to detect the position of the beam at the point of impact. However, when these backscattered electrons bombard the electron detector diodes, these detector diodes themselves emit more backscattered electrons and also secondary electrons of lesser energy, both of which are directed down onto the substrate because of the close proximity of the substrate to the detector diodes. The backscattered electrons are able to penetrate the resist but the secondary electrons, because of their lesser energy, are deposited on the resist, which is non-conducting, and cause a charge buildup on the resist which in turn can deflect the electron beam. This means that the electron beam may not be bombarding the substrate where it is supposed to and this leads to pattern distortion.
It is therefore an object of this invention to reduce or eliminate pattern distortion caused by secondary electrons emitted by the electron detector diodes used in an electron beam lithography machine.